<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>ele</title>
</head>
<body>
    <script>
        let docEle = document.documentElement;
        function setRemUnit () {
          //750/10=75   375/10=37.5
          docEle.style.fontSize = docEle.clientWidth / 10 + 'px';
        }
        setRemUnit();
        window.addEventListener('resize', setRemUnit);
      </script>
      <div id="app">111</div>
    
</body>
</html>